Small asteroids are probably young

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Simulation of the SCI impact. a) - c) Snapshots of the simulation at different tSimulation of the SCI impact. a) - c) Snapshots of the simulation at different times. At t = 1200s, the development of the crater is over. d) SCI crater on the asteroid Ryugu. The key characteristics of the observed crater, including the displacement of the boulders, are recreated in the simulation. @ Courtesy of Martin Jutzi

The impact experiment conducted on the asteroid Ryugu by the Japanese Hayabusa2 mission which took place two years ago resulted in an unexpectedly large crater. With the use of simulations, a team led by the University of Bern and the National Center of Competence in Research (NCCR) PlanetS has recently succeeded in gaining new insights from the experiment regarding the formation and development of asteroids. These insights are also important for the DART mission of NASA.

The Hayabusa2 spacecraft was developed in order to study the history of the asteroid Ryugu, and collected samples and returned them to earth for laboratory analysis. The project participants are Dr. Martin Jutzi and Dr. Sabina Raducan, both from the Physical Institute of the University of Bern, Department for Space Research and Planetology (WP), and are members of the National Center of Competence in Research (NCCR) PlanetS. Under their leadership, in a study which has recently been published in Nature Communications, the team has presented new findings on the formation and development of asteroids.

Rules on the development of craters help with dating asteroids

To explore the characteristics of asteroids, during the space mission Hayabusa2, a Small Carry-on Impactor was fired at the surface of the asteroid Ryugu. "The crater made by the impact was far larger than expected. We therefore tried to reproduce the results of the impact on Ryugu with the use of simulations, to ascertain the kind of characteristics the material is required to have on the surface of the asteroid," explains Martin Jutzi.

The nature and the size of an impact crater on an asteroid are influenced by various factors. Firstly, by the specific characteristics of the projectile, and secondly, by the characteristics of the asteroid—its strength or gravity, for example. "The size and nature of the crater resulting from the impact can lead to a direct diagnosis of the material characteristics and the near-surface structure of the asteroid," explains Jutzi. The study of the crater formation process therefore has important implications for the understanding of the geological and geophysical development of asteroids.

"So far, the way in which the formation of craters works at low gravity has largely remained unexplored. This is because the conditions of the impact cannot be simulated in laboratory experiments on Earth," explains Sabina Raducan, who is managing the project together with Martin Jutzi. The researchers show that the asteroid probably has a very loose internal structure and is only held together by very small cohesive forces and gravitational interactions. "On the basis of these conditions, we are able to use our numerical simulations to reproduce the outcome of the impact on Ryugu," explains Raducan.

The relationships between the characteristics of the projectiles and the size of the crater derived from the results indicate that the surfaces of small asteroids must be very young. "Our results also show that low cohesion can have a significant impact on crater formation. On Ryugu, there are various geological surface units that have different ages. This may be attributable to the influence of cohesion," adds Jutzi.

Important findings for DART

The work of Jutzi and Raducan is also important for the "Double Asteroid Redirection Test" (DART) mission by NASA, in which the scientists are also taking part. DART is the first full test in the world regarding planetary defense against the possible impact of asteroids on Earth. On 27 September 2022, as part of the DART mission a space probe crashed into the asteroid Dimorphos to deflect the asteroid from its orbit. "The findings of the simulations for the impact on Ryugu also help with analyzing the results of the DART mission", explains Jutzi. "We are working on applying the newly developed models to DART in order to gain insights into the characteristics of Dimorphos. Our initial simulations look very promising," adds Raducan.